The carrier said the number of passengers dropped to 9.5 million last month – the lowest since July and down from 10.2 million in November.
It ran 62,200 flights last month, with a load factor – a measure of how well an airline fills its planes – of 81%, down from 86% in November.
The group had warned over profits just before Christmas as it cut its passenger forecast for December and January due to Omicron restrictions.
The group had previously expected to fly between 10 and 11 million passengers in December, while it also slashed its guidance for January by 33% as restrictions were imposed on British passengers flying to Germany and France and all EU passengers to Morocco.
It slashed its full-year results guidance to a net loss of between 250 million euros (£209 million) to 450 million euros (£375 million) from a previous net loss range of between 100 million euros (£83 million) to 200 million euros (£167 million).
On a year-on-year basis, Ryanair saw passenger numbers rise from 1.9 million in December 2020, when there were strict restrictions to control last winter’s wave of the pandemic.
The UK Government moved to so-called Plan B restrictions last month to control the spread of Omicron, instructing people to work from home and increase mask wearing, while some travel curbs have also been imposed in a number of countries across Europe.
But the Government resisted further pandemic restrictions over the festive period and Prime Minister Boris Johnson said on Tuesday he hoped to “ride out” the wave of Omicron without lockdown measures despite the NHS coming under significant strain.